1. Field of the Invention
This invention relates to an operating mechanism for a circuit interrupting device, and more specifically relates to a simple, reliable, inexpensive, and reusable line-potential operating mechanism for a bus-mountable, manually-resettable, circuit interrupting device, which device may be used in circuits in which faults may exceed the interrupting capability of an interrupting unit of the device.
2. Prior Art
Various types of circuit interrupting devices--including fuses or fuse-like devices, circuit breakers, reclosers, and circuit switchers--are well known for circuit protection. These devices have varying operational characteristics and features which make their use technically attractive in some environments, but technically less attractive in others. In the past, operational features tended to be the only criteria determining which type of device was to be used. Today, however, the relative cost of the devices is becoming an important, if not the most important, determinant in deciding which device shall be used in a particular environment. Users of these devices are today often willing to forgo purchasing exotic, broad-range interrupting capability devices in favor of inexpensive, simple devices, even though the latter may have more limited interrupting capabilities.
One use environment in which users today are inclined to employ less expensive, simple devices is that of transformer protection. Devices which up to now have been employed for transformer protection have been either (1) complicated or expensive or both, or (2) if simple or inexpensive, not re-usable. Economic conditions have forced equipment users, especially utilities, to consider employing transformer-protective devices which are not broad-range, and are therefore less versatile or sophisticated, but which are at the same time substantially less expensive. Specifically, many utilities have come to realize that there are times when it is expeditious to use an inexpensive interrupting device having a more limited interrupting rating, as opposed to a broad-range interrupting device, if the inexpensive device is sufficiently less expensive than alternative broad-range devices, so as to make the use of the former attractive from a capital investment standpoint. The attractiveness of such inexpensive devices is enhanced if they are re-usable. Thus, one object of this invention is the provision of a reliable, inexpensive, simple and re-usable interrupting device having limited interrupting capability.
If a broad-range device, such as a circuit breaker, is used as the sole device protecting a transformer, several potential negative aspects, in addition to its high cost, may be present. First, the impedance of the transformer and of the conductors between the transformer and the breaker may so limit currents on the transformer's primary caused by secondary faults that the breaker does not timely respond. Second, because of their high cost, it is often expedient to apply breakers to protect several branch circuits fed by a larger transmission or distribution circuit. In this event, operation of the breaker due to a secondary fault in the transformer located in one such branch circuit de-energizes all such branch circuits. Thus, a fault in one part of a system may render inoperative a large portion of the system. If the breaker is moved closer to the transformer and does not involve branches other than the one in which the transformer is located, it may be under-utilized, having the ability of more extensive system protection. Such under-utilization is unattractive from a cost standpoint.
Placement of a relatively cheap protective device--such as a fuse--intermediate the breaker (protecting several branches) and the transformer (in one of the branches) is an obvious expedient. However, until recently, choices of such cheaper devices have been limited. Moreover, where fuses are used, they must be replaced or replenished following performance of their protective function. Fuse-like devices which are reliable, relatively inexpensive and partly re-usable have only lately become available. See, for example, the following commonly-assigned, U.S. patent applications: Ser. Nos. 909,144 and 909,145 in the names of O. Meister and T. J. Tobin; and Ser. No. 909,146, now U.S. Pat. No. 4,161,711 in the name of O. Meister; all filed on May 24, 1978. Nevertheless, another object of the present invention is to provide a reliable, inexpensive, and re-usable limited fault-interrupting capability device as an alternative to fuses, fuse-like devices and breakers for placement between a transformer and a more expensive, broad-range interrupting device.
Most prior art interrupting devices include various types of "intelligence" and sensing schemes. The "intelligence" processes information provided by the sensing, which information relates to the current flowing in an electrical circuit. The "intelligence" dictates the operation or lack of operation of the device based on such information. Various sensing and "intelligence" schemes are well known; some are simple, while others are quite complex. One common characteristic of many present-day transformer protective devices, other than fuses, is the location of the "intelligence" at ground potential while the sensing and the device are at line potential. This location, of course, necessitates electrical insulation and isolation between the "intelligence," on the one hand, and the sensing and the device, on the other hand. This requirement for isolation, in turn, leads to the use of complex mechanical and electrical schemes for interconnecting the sensing and the "intelligence" to each other and to the circuit interrupting device.
Accordingly, it is desirable to provide a simple, reliable, inexpensive interrupting device, having a limited interrupting rating, but which nevertheless is attractive in view of its low cost, which is completely bus- or line-mountable, and which operates entirely at bus or line potential. Such a device is even more attractive should its entire sensing and "intelligence" be at line or bus potential, thus obviating the need for complex interconnections between the interrupting device and its "intelligence" and sensing. From a cost standpoint, it is also desirable that such a device be re-usable and manually resettable from the ground to obviate the necessity of expensive and complicated reclosing mechanisms. Toward these ends, the present invention is aimed.
The present invention is also aimed at providing a simple, reliable, low-cost operating mechanism for a variety of known interrupting units, wherein fault-interrupting ability is achieved at low cost.